The gradient refractive-index (GRIN) rod lens is a cylindrical medium with a parabolic refractive distribution in which the refractive index is highest on the rod's optical axis and decreases toward the periphery with the square of the radial distance from the optical axis. GRIN elements with imaging and light-focusing properties have been used widely in image-transmission systems [Y. Koike, "Gradient index materials and compounds", L. A. Hormak, Ed., Marcel Dekker, Inc., New York 1992; Y. Koike, T. Ishigure, A. Horibe, E. Nihei, Proceedings of second International Conference on Plastic Optical Fibers and Applications, Geneva, p. 54, 1993; H. Tsuchida, T. Nagaoka, K. Yamamoto, Jap. J. Appl. Phys. Part 1, 37, 3633 (1998)] and optical communication systems [J. Wilson, J. F. B. Hawkes, "Optoelectronics: An introduction", Prentice Hall, New Jersey, p. 357-377, 1983; G. Stewart, A. Mencaglia, W. Philip, W. Jin, J. Lightwave Tech. 16, 43 (1998); S. H. Song, S. Park, C. H. Oh, P. S. Kim, M. H. Cho, Y. S. Kim, Opt. Lett. 23, 1025 (1998); T. Fukushima, T. Yokota, T. Sakamoto, J. Lightwave Tech. 15, 1938 (1997); A. Safaaijazi, V. Suppanitchakij, IEEE J. Ouantum Electro. 33, 2159 (1997)] such as copy machines, facsimile lens arrays, and optical circuit networks. Glasses and polymers are the two well-known materials applied in the optical field. The former have excellent transparency and low optical attenuation, but brittleness and high process cost are their disadvantages. The latter have higher optical loss but excellent mechanical properties, light weight, good flexibility, easy processing, and low cost. As a result the development of GRIN polymers has grown rapidly in recent years [S. P. Wu, E. Nihei, Y. Koike, Polym. J. 27, 21 (1995); C. Wang, D. L. Shealy, Appl. Opt. 32, 4763 (1993)].
Gradient refractive index (GRIN) optical rods with a quadratic refractive index distribution varying continuously from the optical axis to the periphery have been widely studied because of their potential application in self-focusing imaging and optical communications recently. Several methods have been used to prepare GRIN polymeric optical rods: two-step copolymerization [Y. Ohtsuka, Y. Terao, J. Appl. Polym. Sci. 26, 2907 (1981); Y. Ohtsuka, T. Sugaho, Appl. Opt. 22, 413 (1983)], the extrusion method [B. C. Ho, J. H. Chen, W. C. Chen, Y. H. Chang, Polym. J. 27, 310 (1995)], interfacial-gel copolymerization [Y. Koike, Y. Kimoto, Y. Ohtsuka, Appl. Opt. 21, 1057 (1982)], and photopolymerization [J. H. Liu, M. H. Chu, Angew. Macromol. Chem. 174,1 (1989)].
In a series of the studies of one of the present inventors and his co-workers, on the GRIN rods, a method of swollen-gel polymerization for preparing the large core and bubble-free GRIN polymer optical rods was reported [J. H. Liu, H. T. Liu, Opt. Lett. 22, 668 (1997); J. H. Liu, H. T. Liu, Macromol. Chem. Phys. 198, 3285 (1997); J. H. Liu, U.S. Pat. No. 5,846,456, December 1998; J. H. Liu, H. T. Liu, Y. B. Chen, Polymer, 39, 5549 (1998)]. In the swollen-gel polymerrization method, a comonomer mixture is injected into a preformed polymer tube, and the polymerization of the comonomer mixture will be initiated after the preformed polymer tube is swelled by the comonomer mixture for a period of time. The preformed polymer tube has to be uniform in thickness and free from substantial deformation during swelling in order to ensure that the GRIN polymer optical rod so fabricated has desired optical characteristics. It is difficult to make such a preformed polymer tube that it not only can be swelled by the comonomer mixture, but is uniform in thickness and free from substantial deformation during swelling. Consequently, the reproducibility of the GRIN polymer optical rod fabricated by the swollen-gel polymerization method is not satisfactory, and thus there is a need to provide an improved method for fabricating the GRIN plastic rod, which is easy in operation and has a high reproducibility.